1. Field of the Invention
The present invention generally relates to lithography templates. More particularly, certain embodiments of the invention relate to the formation of imprint lithography templates fabricated by pattern generators.
2. Description of the Relevant Art
For imprint lithography, a relief pattern in a template is used, in conjunction with monomers/polymers to imprint a desired pattern into monomers/polymers on the surface of a substrate (e.g., semiconductors, dielectric materials, magnetic or optoelectonic materials.) The processes commonly used to manufacture projection photomasks are often applied to the manufacture of templates for imprint lithography. For this reason, it is useful to provide background of the manufacture of optical projection lithography and optical projection photomasks.
A conventional projection photolithography system projects a UV light pattern onto a light sensitive coating (e.g., a photoresist) to expose selected portions of the light sensitive coating. The light sensitive coating is developed to create a mask for a fabrication process such as etching or doping of the underlying substrate. The photolithography systems commonly employ a photomask or reticle that controls which portions of the light sensitive coating are illuminated. For integrated circuit manufacturing, the photomask has a precise pattern that the projection transfers, with or without demagnification, to the integrated circuit device.
Photomask making begins with an optically transparent substrate (e.g., quartz). One side of the transparent substrate is typically coated with an optically opaque film of a material (e.g., chromium). A resist material (e.g., a polymer) layer is then applied to the opaque film, and a pattern generation process exposes the photoresist layer to light or electron bombardment. Various types of pattern generation equipment are known. For example, scanning systems may be programmed with a digitized image or pixel pattern that corresponds to the desired pattern to be exposed on the photoresist layer. The scanning system exposes only the photoresist areas that correspond to the pixels having values indicating that the areas should be exposed.
Developing of the photoresist layer creates a photoresist pattern with openings that expose the underlying opaque layer. The photoresist pattern and openings have a critical or minimum feature size that depends on the pattern generation equipment used to expose the photoresist layer. An etching process, typically an anisotropic etch, using the photoresist pattern as an etching mask removes portions of the opaque layer to create an opaque pattern having openings. Etching of the opaque material may be difficult if the opaque material is a metal. Many metals produce particles and aggregates during a dry etch process which may be deposited on the substrate creating defects in the pattern. A wet etch process may be used to avoid the deposition of particles, however, wet etching processes suffer from undercutting problems for very small features. Following the etch, the photoresist pattern is stripped from the substrate, leaving a hard photomask that includes a discontinuous opaque pattern on the substrate. The photomask is then measured, inspected and repaired if necessary. The opaque pattern provides a high contrast binary image for the projection of the photomask in a photolithography system. Alternatively, the opaque layer may be removed to form a set of openings in the underlying transparent substrate. Such a process is used to create a “phase mask.” The depth of the openings formed on the transparent substrate are chosen to maximize the phase contrast at the exposure wavelength. Typically, a phase mask is inspected at the exposure wavelength to obtain the maximum contrast.
A problem with trying to apply photomask manufacturing process for the manufacture of imprint lithography templates is that the completed photomask tends to have a critical feature size that is generally larger than the feature size that the pattern generation equipment can create. In particular, the etching process using the photoresist pattern as etch mask is often a wet chemistry etch (or isotropic) process. An isotropic, wet chemistry etch process has historically been desirable because a wet etch process is inexpensive and relatively defect free. However, the isotropic etching undercuts the photoresist pattern by about the thickness of the opaque layer or more and makes the openings in the opaque pattern larger than the original openings in photoresist pattern. For previous generations of semiconductor devices, the undercutting, while not desirable, was acceptable. However, as feature sizes become smaller, the size of the undercut becomes more difficult to accommodate, and higher resolution generations of integrated circuits having smaller feature sizes have found the undercutting unacceptable.
To overcome this difficulty, the use of anisotropic etch of the opaque material has been explored. Heavy metal compounds that are liberated during the dry etch processes, however, inherently accrete and precipitate to create defects on the photomask surface. Additionally there tends to be relatively poor etch selectivity (between the photoresist pattern and the opaque layer) that results in some undercut because the dry etch widens the openings. Despite these difficulties, dry etching processes for the creation of high resolution photomasks are generally preferred despite the additional costs of defect repair and lower yields.
One method to avoid heavy metals, is to coat the photoresist directly onto the substrate and then to coat the photoresist with a conductive top coat (e.g., Aquatar). The conductive top coat will bleed a charge to facilitate high resolution e-beam patterning of the photoresist. The photoresist will act as the etch mask for etching a relief pattern into the quartz substrate with anisotropic, high selectivity etching and substantially no undercut.
Another method to eliminate heavy metals and still remove charge during e-beam patterning was reported by D. J. Resnick at the SPIE's 27h Annual International Symposium and Education Program on Microlithography, Mar. 3-8, 2002 in Santa Clara, Calif. By incorporating a permanent conductive layer of indium tin oxide on a substrate, charge bleeding is facilitated, not only during pattern generation, but also at subsequent inspections which may also use electron beams. However, this technique has drawbacks. The indium tin oxide layer, while transparent at visible wavelengths, is generally opaque at deep ultraviolet wavelengths thereby limiting the use of deep ultraviolet wavelengths in imprint lithography that would use such templates.
Imprint lithography templates also tend to have a much higher aspect ratio than photomasks. Thus, the depth of an imprint lithography template is typically greater than a depth of a photomask. The greater depth of the recesses in an imprint lithography template may make inspection difficult.